One form of powertrain transmissions utilizes a multiple clutch system to transmit torque from an input source, such as an engine or motor, to a gear box or other gear shifting mechanism, which in turn transfers torque and rotational speed to a drivetrain. Such multi-clutch systems may be used in manual, an assisted manual or automatic transmissions. One type of multi-clutch system is a dual clutch transmission such as those used in automotive applications. Dual clutch transmissions typically are provided with a single torque input from the engine that is transferred by the clutch to one of a pair of shafts out of the clutch to the transmission gear box.
The torque input is transferred to the output shafts through a pair of selectively engageable, compressible sets of clutch friction elements, such as stacks of clutch disks. One example of such a dual clutch system has a first and a second clutch friction element or clutch stack, and, where disk stacks are used, each stack has a set of driving disks and a set of driven disks. The driving disks are operatively connected to the torque input, and the driven disks are operatively connected to one of the output shafts. In the gear box, the output shafts provide torque to different gear sets, providing different gear ratios to the drivetrain.
In such a system, one of the clutch stack/output shafts can provide torque to the even numbered gear ratios in the transmission and the other shaft can provide torque to the odd numbered gear ratios and a reverse gear. Other multi-clutch systems typically provide a similar arrangement of multiple clutch friction elements providing torque transfer to preselected output shafts/gear ratio combinations.
By selectively operating the clutch friction elements, the operator or operating system can frictionally engage the driving elements and the driven elements to transmit torque to preselected gear ratios. The amount of torque transfer will depend on the degree to which the driving and driven friction elements are engaged, the engine speed and other related factors. Because one set of friction elements is engaged while the other set(s) is inactive, additional gear ratios may be selected in the transmission and engaged to the output shaft(s) connected to the inactive clutch friction elements. The gear shift is accomplished by disengaging the active set of friction elements and activating the selected inactive set that is already engaged with a new gear. Thus, the time required to shift to the new, pre-selected gear ratio can be reduced, clutch engagement and disengagement interruptions can be reduced, and a smoother gear shift can be accomplished.
In “wet” clutch systems, a consistent flow of an oil, transmission fluid or other lubricating fluid, also is maintained through the clutch. The lubricating fluid flows through supply channels and through the clutch friction elements providing lubrication to the friction elements, seals and other moving parts. This fluid flow reduces friction wear on the friction elements and further serves an important role in cooling the clutch and the friction elements.
Many multi-clutch systems position the sets of friction elements, e.g. clutch disk stacks, radially with respect to each other. In other systems, the sets of clutch friction elements are positioned parallel to each other along the principal axis of rotation of the clutch mechanism. Other arrangements also may be used depending on the number of sets of clutch friction elements, space concerns, efficiency concerns, etc.
In many multi-clutch systems, the clutch output shafts are concentrically arranged with respect to each other. One example of such an arrangement in a dual clutch system uses a first inner clutch output shaft connected to one of the clutch disk stacks and positioned within a hollow second, outer clutch output shaft, that is connected to the other clutch disk stack. The selective activation of either the first or the second clutch stacks allows for the torque input from, for example, an engine drive shaft to one of the inner or outer output shafts.
In such a system, each clutch stack can be hydraulically activated by radially extending annular pistons. The pistons often extend from a location proximate a clutch support to the outer clutch plates of each of their clutch stacks. The pistons together with an annular cylinder and/or inner walls of the clutch define a pressure chamber for each piston. When a flow of fluid (typically transmission oil) is applied to the pressure chamber and thus to one of the pistons, the piston contacts the clutch stack with a force sufficient to compress and frictionally engage the discs of the clutch stack. In such systems, accordingly, when one stack is engaged the other is inactive, and shifts are discrete events made as quickly as possible
A microprocessor controller frequently is used to operate the clutch systems, and alone or in co-ordination with one or more other controllers such as those directing the selection of specific gear ratios in the transmission, engine controllers, etc. For example, a microprocessor controller may be used to direct the application of pressure to the disk stacks, the supply of cooling and lubrication fluids to the disk stacks, and the shifting of gears in the gearbox, etc.
The overall performance of a transmission often is evaluated in several respects. Some factors involve objective measurements, such as torque transfer efficiency, shift time, the clutch and/or gear endurance and durability, shift efficiency and duration, potential fuel savings, etc. Other factors are more subjective, such as shift noise, shift busyness (i.e. number of shifts for a given time period/condition), shift timing, shift stiffness, etc. In manual transmissions, many of these considerations are subject to the operator's direct control. In automatic transmissions and partially automatic transmissions factors such as shift timing, shift busyness, etc. are subject to the controller system, the gear ratios, and other mechanical or electromechanical systems in the transmission.
The balancing of these factors often results in compromises necessary to satisfy specific design criteria, cost considerations and criteria unrelated to the function and operation of the transmission. For example, automatic transmissions can require a relatively broad range of gear ratios to provide adequate load bearing capacity at a vehicle start up using a first or second gear, and to provide high speed travel capabilities at a fourth, fifth or sixth gear.
Standard manual transmissions, and some semi-manual transmissions, with a comparable range of gear ratios often require undesirable, rapid shifting at relatively low vehicle speeds, particularly when the vehicle is launched from a stop or near launch speeds. This undesirable “shiftiness” also can be present in dual clutch transmissions with such a range of gear ratios. Such undesirable “shiftiness,” in addition, can occur where a manual transmission is replaced by a dual clutch automated transmission or other automated transmissions.
Accordingly, in many multi-clutch systems, the gear ratios necessary under heavy load conditions, such as towing a trailer, are unnecessary under no-load conditions, but nevertheless are used to satisfy the operational demands for the transmission. For example, when the vehicle is launching, or starting from rest, the high ratio first gear often is needed only for a brief time, normally for seconds, and then the transmission typically is shifted to the next, higher gear ratios. A similar shifting occurs when a vehicle under load (e.g. from cargo or towing a trailer) is traveling at a very slow speed and rapid acceleration is required.
Such conditions can produce an undesirable “busy” shifting routine, particularly if the transmission controller senses changing loads or speeds requiring shifting repeatedly between a first gear to a second or higher gear. The resulting noise and vibrations induced due to such shifting operations is undesirable for many applications.
One approach to addressing this issue has been to use a higher gear (and lower gear ratio) for vehicle start up or launch conditions, and for similar conditions requiring greater torque transfer to a drive train. This approach, however, is unsuitable for many applications, particularly those where the drive train load conditions and torque needs are variable, and where the vehicle/drive train loads may cause engine stalling or damage at those gear ratios.
The operating inefficiencies that result from the use of gear ratios that are not well matched to specific driving conditions, i.e. high gear ratios in first or second gears, also can result in undesirable losses in fuel economy, transmission and/or powertrain durability, etc. For example, at start up and thereafter, unnecessarily high gear ratios will result in use of excessive engine speeds to achieve desired rates of speed or speed increases. Similarly, the use of gear ratios that are too low for a particular load and vehicle condition will result in engine inefficiencies causing a reduction in fuel efficiencies and potential engine damage.